1. Field of the Invention
The present invention relates to a balance with hairspring mounted in a timepiece, a movement including the balance with hairspring, and a timepiece.
2. Description of the Related Art
As a mechanism for adjusting the rate of a mechanical timepiece, a free sprung balance is known (See, for example, U.S. Pat. No. 7,661,875 (Patent Literature 1)). The free sprung balance is a mechanism that varies the moment of inertia of a balance with hairspring to vary the vibration cycle thereof, thereby adjusting the losing and gaining of the timepiece. The vibration cycle of the balance with hairspring can be expressed by equation (1).
[Math. 1]
                    T        =                  2          ⁢                                          ⁢          π          ⁢                                    I              K                                                          Equation        ⁢                                  ⁢                  (          1          )                    
When the moment of inertia of the balance with hairspring is larger than that expressed by equation (1), the vibration cycle of the balance with hairspring is long; and when the moment of inertia of the balance with hairspring is smaller than that expressed by equation (1), the vibration cycle of the balance with hairspring is short. As a method of varying the moment of inertia, there is generally used a weight; by moving the weight in the radial direction, the moment of inertia is varied.
In this conventional technique, the positions of a plurality of screwed weights are separately adjusted, so that if the movement amount of each weight differs even if slightly, the center of gravity of the balance with hairspring is deviated from the rotational axis, and there is a fear of the vibration cycle of the balance with hairspring in the upright attitude being disturbed.
Further, there is known a mechanism in which the positions of weights are moved along arms also serving as a guide to thereby adjust the moment of inertia (See, for example, U.S. Pat. No. 2,880,570 (Patent Literature 2)). The positions of the weights are regulated by a weight position regulation member; by rotating this component with respect to the balance staff, it is advantageously possible to move the weights situated opposite the rotational axis by the amount.
In the conventional technique, however, there cannot but exist, from the viewpoint of manufacture, a clearance between the weight and the guide rail and between the weight and the weight position regulation member; and, due to this clearance, there is the possibility of the positions of opposite weights being deviated. When the positions of opposite weights are deviated, the center of gravity of the balance with hairspring is separated from the rotational axis, and there is attained a single-sidedness in weight, in which the position of the center of gravity is single-sided, so that there is a fear of the vibration cycle of the balance with hairspring in the upright attitude being disturbed.